A glacier is a large body of dense ice that forms on land and moves gradually under the influence of its own immense weight and gravity. These colossal masses of frozen water are powerful geological agents capable of reshaping mountains and valleys over great stretches of time. Glacial erosion is the process by which this moving ice grinds, scrapes, and plucks rock material from the Earth’s surface, resulting in some of the most dramatic topographic features found across the planet.
Glacial Abrasion: The Grinding Action
Glacial abrasion is a mechanical process where the moving ice acts much like a giant piece of sandpaper, relentlessly wearing away the underlying bedrock. Pure ice is relatively soft and would not be an efficient erosive tool on its own. However, glaciers incorporate vast amounts of rock fragments and sediment, often referred to as debris, into their base and lower layers.
These entrained rocks, ranging from silt to large boulders, become frozen into the ice and are dragged across the rock surface as the glacier slides forward. The weight of the overlying ice, which can be hundreds or thousands of feet thick, presses these embedded fragments down onto the bedrock with enormous pressure. This force causes the fragments to scrape, scour, and polish the rock surface below.
A clear signature of this process is the formation of glacial striations, which are long, parallel scratches or grooves etched into the bedrock, indicating the direction of ice flow. Abrasion also produces rock flour, an extremely fine, silt-sized powder created by the continuous grinding action. The presence of rock flour suspended in the water gives glacial meltwater rivers and lakes a characteristic milky, opaque turquoise color.
Plucking and Quarrying: Removing Bedrock
The second major mechanism of glacial erosion is plucking, also known as quarrying, which is responsible for the removal of larger blocks of rock. This process is particularly effective where the bedrock is already weakened by pre-existing joints, fractures, or bedding planes. Plucking occurs when basal meltwater, generated by the pressure and friction at the bottom of the glacier, seeps into these cracks in the underlying rock.
As the meltwater refreezes within the fractures, it undergoes a volume expansion of approximately nine percent, exerting immense force on the rock walls. This action, known as frost wedging, progressively weakens and enlarges the fissures. The sheer weight and forward motion of the overriding glacier then pull away the loosened, fractured blocks of rock from the bedrock surface.
These newly detached joint blocks, which can measure several meters across, are then incorporated into the base of the glacier. Once entrained, these plucked blocks themselves become powerful tools that enhance the abrasive action against the downstream bedrock.
Variables Determining Erosive Force
One of the most significant variables is the glacier’s velocity, or sliding speed, which is the rate at which the ice moves over the bed. Faster movement means that the abrasive tools at the base pass over the bedrock more frequently, increasing the rate of grinding.
Basal temperature also plays a determining role, as it controls the presence of meltwater at the ice-rock interface. Warm-based, or temperate, glaciers have a thin layer of meltwater at their base, which facilitates the sliding motion and is necessary for both plucking and efficient abrasion. Conversely, cold-based glaciers are frozen to the bedrock and move primarily by internal deformation, resulting in far lower rates of erosion.
The concentration and type of debris load within the basal ice also directly influence abrasive force; a glacier with more embedded rock fragments will be more effective at grinding than one with clean ice. Finally, the thickness of the ice sheet or glacier dictates the pressure exerted on the bed, which enhances the crushing effect of the abrasive tools and contributes to the pressure melting required for plucking and basal sliding.
Distinctive Landforms Created by Glacial Erosion
The sustained action of abrasion and plucking sculpts the landscape into a suite of highly distinctive features. One of the most recognizable landforms is the U-shaped valley, or glacial trough, which is created when a glacier widens and deepens a pre-existing, V-shaped river valley. The broad, flat floor and steep, straightened sides are the result of the ice mass eroding the valley floor and interlocking spurs equally.
Cirques are bowl-shaped, amphitheater-like depressions found high on mountain sides, marking the starting point of mountain glaciers. These hollows are carved out by the rotational movement of ice and intense freeze-thaw weathering, which combines plucking at the headwall with abrasion on the floor. When two or more cirques erode back-to-back on opposite sides of a mountain, they can form a narrow, jagged, knife-edge ridge called an arête.
The selective application of plucking on fractured rock and abrasion on smoother surfaces results in streamlined bedrock features. These features include roches moutonnées, which are rounded on the upstream side by abrasion and steep and rough on the downstream side due to plucking.